1. Field of the Invention
The present invention relates generally to a film bulk acoustic resonator, and more particularly to a film bulk acoustic resonator with improved lateral mode suppression.
2. Description of the Prior Art
Currently, there increases a demand for Radio Frequency (RF) filters that are mounted in portable communication systems having a carrier frequency bandwidth of 900 MHz to 3 GHz. RF filters are generally classified into dielectric filters and Surface Acoustic Wave (SAW) filters. In recent years, a Film Bulk Acoustic Resonator (FBAR) using semiconductor thin film technology has been developed to implement a single chip.
FIGS. 1a and 1b show a general structure of such a FBAR that is manufactured by the thin film technology, in which FIG. 1a is a front sectional view thereof and FIG. 1b is a plan view thereof. The FBAR 10, as shown in FIGS. 1a and 1b, comprises a substrate 11, a piezoelectric layer 12 positioned on the substrate 11 and made of a piezoelectric material such as ZnO or AlN, and metallic electrodes 13 and 14 attached to the upper and lower surfaces of the piezoelectric layer 12. The FBAR functions to pass therethrough only a signal around a resonant frequency adjusted by the thickness of the piezoelectric layer while part of electric energy inputted to the piezoelectric layer 12 through the electrodes 13 and 14 is converted into mechanical energy and this mechanical energy is reconverted into electric energy.
The FBAR utilizes transverse resonant characteristics. Ideally, it is preferable that only transverse modes exist. As shown in FIG. 2, a frequency pass characteristic graph for a filter comprised of FBARs, in which lateral modes are generated, represents characteristics that an abrupt peak is generated in a pass characteristic graph around a pass band, its characteristic curve is not smooth, and prominences and depressions are generated like the generation of noises. Generally, the thickness of the FBAR is about 0.5 to 3.0 xcexcm, and the width of the piezoelectric layer is about 30 to 300 xcexcm. In the FBAR, the ratio of the thickness to the width is about {fraction (1/500)} to {fraction (1/50)}. The thickness is considerably small compared with the width of the piezoelectric layer. If the ratio of the thickness of the piezoelectric layer to the width is small, the lateral modes in the piezoelectric layer are generally negligible. However, it the ratio of the thickness to the width is increased by decreasing the width to reduce the area of the FBAR, the increase in the ratio affects the transverse vibration characteristics of the piezoelectric layer. In particular, as shown in FIG. 1b, when both opposite sides of an electrode 13 of the FBAR 10 are parallel with each other, lateral modes 15 generated on one of the opposite sides are reflected by the other side and superposed on one another. If the lateral modes are superposed on one another and amplified, a lateral mode having an exceedingly small amplitude affects transverse vibration characteristics.
In order to suppress such lateral modes, U.S. Pat. No. 6,150,703 discloses a FBAR 20 in which the sides of its piezoelectric layer 22 or electrodes 23, as shown in FIG. 3, are comprised of straight lines that are not parallel with one another.
Additionally, U.S. Pat. No. 6,215,375 discloses a FBAR 30 in which the sides of its electrode 33, as shown in FIG. 4, forms an irregular polygon whose any two sides are not parallel with each other. This FBAR is effective in suppressing lateral modes 35 in such a way that the lateral modes 35 are prevented from being superposed on one another.
However, in the conventional FBARs, all the sides of their electrodes are comprised of straight lines, so the interference of neighboring modes is small. In more detail, as shown in FIG. 3, modes generated on one side are reflected by the opposite side and the latter side forms a straight line, so the interference of lateral modes is reduced. Accordingly, lateral modes, which are not destroyed by interference, exist and affect the transverse vibration characteristics of the piezoelectric layer.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a FBAR, which is capable of suppressing lateral modes, thereby having smooth characteristics without irregularities.
Another object of the present invention is to provide a pass band filter in which a plurality of FBARs are connected to one another.
In order to accomplish the above object, the present invention provides a film bulk acoustic resonator, comprising a piezoelectric layer, and electrodes formed on both surfaces of the piezoelectric layer to be partially opposite and overlapped, wherein all sides of at least one of the electrodes and/or all sides of the piezoelectric layer are curved.
In addition, the present invention provides a film bulk acoustic resonator, comprising a substrate; a supporting layer formed on the substrate to form an air gap; at least two piezoelectric layers formed on the supporting layer; and a pair of electrodes formed on both surfaces of the piezoelectric layer to be partially opposite and overlapped; wherein all the sides of at least one of electrode and/or all the sides of at least one of the piezoelectric layers are curved.
In addition, the present invention provides a filter that is constructed by connecting a plurality of FBARs in parallel or series with one another.